In recent years, the American Institute of Electrical and Electronics Engineers has proposed a standard by the name of IEEE802.3af. This is a standard for a circuit equipping a feed circuit on the conventional signal transmission circuit, which is a standard for feeding also an electric power through a signal line, such as a LAN cable, for transmitting and receiving signals.
FIG. 1 is a diagram showing an eight-line LAN cable 1, for use in the standard, which is connected to a connector 2 (e.g., RJ45 type connector 2), where the pin numbers “1, 2, 3 and 6” are used for a data transmission and also for feeding power. When feeding, the pin numbers “1 and 2” are used for applying a voltage up to 48 volts, while the pin numbers “3 and 6” are used for a ground (GND) voltage.
Meanwhile, the pin numbers “4, 5, 7 and 8” are spared, except for the case of using them for a data transmission and feeding power.
FIG. 2 is a diagram showing a system utilizing the above noted LAN cable 1. A communication apparatus A communicates with a communication apparatus B and also feeds power thereto. Taking the communication apparatus A as a feed apparatus 3 and the communication apparatus B as a power reception apparatus 4, the power reception apparatus 4 for use as per the IEEE802.3af commonly comprises a incoming circuit 4-1, a PHY 4-2 and a field programmable array gate array (FPGA) 4-3 as shown in FIG. 3.
The incoming circuit 4-1 is a circuit for receiving a power supplied from a feed apparatus.
The PHY 4-2 is a circuit for converting a data format handled by a physical layer of an OSI layer model into a data format which is handled by a data link layer. The OSI layer model is a hierarchical layer structure model for a communication function based on the Open Systems Interconnection (OSI) which is a design policy of a network structure established by the International Organization for Standardization (ISO).
The FPGA 4-3, being an integrated circuit allowing a change of operations of a logic circuit, is for carrying out data process at the power reception apparatus 4.
In a communication apparatus compliant to the IEEE802.3 Standard supporting a communication and a feed by a single communication cable, the already known is a technique of a complex electric circuit and a highly integrated circuit, e.g., a technique for communication to transmit contact information of the communication apparatus and communication cable to an opposite apparatus with the above noted PHY, FPGA.    [Patent document 1] Laid-Open Japanese Patent Application Publication No. 2000-244373
The conventional technique is actually useful for a transmission using a communication apparatus which carries out a high level communication. However, when considering a relay as a communication apparatus for example, a PHY or an FPGA has been equipped with the communication apparatus which is used as the relay even in the case of only monitoring an abnormality of the relay. That is, in the case of communicating between the first and second communication apparatuses by using a relay, a complex PHY or FPGA has been required for establishing a connection between the relay and a third communication apparatus even when detecting a presence or absence of an abnormality of the relay by the third communication apparatus equipped with a feed function, which is connected to the relay of a feed voltage of an equipment connected to the relay. Equipping a using equipment with these apparatuses levies a large load in terms of design period, evaluation period, cost, et cetera, thus requiring a simple apparatus instead.